Anything with high burst damage, Sion has a nice stun so if you can put out some damage then ignite its almost always a secure kill.

Me and a friend rolled Sion and Blitz (I was Blitz) i'd Q them in and he would stun sombody would instant ignite then other person would ignite once they are running away.
FIRST BLOOD!
everytiem
But anything works with Sion trust me